В спортзал я обычно хожу в толстовке, а не в пиджаке.

Because ходить is the multi-directional / habitual verb of motion. In this sentence, обычно shows a repeated habit: I usually go.

• я хожу в спортзал = I go to the gym / I usually go to the gym
• я иду в спортзал = I am going to the gym right now, in one specific direction

So обычно хожу is the natural choice for a routine.

The preposition в is doing two different jobs here:

1. в спортзал = to the gym
   Here it expresses motion toward a place.

2. в толстовке, в пиджаке = in a sweatshirt, in a jacket/blazer
   Here it expresses what someone is wearing.

Russian often uses в + Prepositional for clothing after verbs like ходить, быть, сидеть, etc.:

• ходить в пальто = to go around wearing a coat
• быть в шляпе = to be wearing a hat

So even though English uses different wording, Russian naturally uses в in both parts.

Because в спортзал shows direction toward the place: to the gym.

Compare:

• в спортзал = to the gym → motion, destination
• в спортзале = in the gym → location

This is a very common Russian pattern:

• идти в школу = to go to school
• быть в школе = to be in school

So the sentence is about where the speaker goes, not where they already are.

It actually is in the accusative case, but for a masculine inanimate noun, the accusative often looks exactly like the nominative.

• Nominative: спортзал
• Accusative: спортзал

So after в meaning motion toward a destination, Russian uses the accusative, but there is no visible change here.

This happens with many masculine inanimate nouns:

• в магазин
• в парк
• в музей

Because after в in the sense of wearing, Russian uses the Prepositional case.

So:

• толстовка → в толстовке
• пиджак → в пиджаке

The endings change according to the noun type:

• feminine nouns in -а / -я often become -е in the Prepositional singular
  толстовка → толстовке
• many masculine nouns also take -е
  пиджак → пиджаке

So the sentence is following a normal pattern:
ходить в чём? = to go around wearing what?

Literally, yes, the image is something like in a sweatshirt, but in normal Russian this is just the standard way to say wearing that item.

Russian often expresses clothing this way:

• Он в куртке. = He is wearing a jacket.
• Она пришла в платье. = She came wearing a dress.
• Я хожу в толстовке. = I go wearing a sweatshirt / I go in a sweatshirt.

So you should treat this as a natural Russian structure, not translate it too literally.

Here а не marks a contrast between two alternatives:

• в толстовке, а не в пиджаке = in a sweatshirt, not in a blazer/jacket

Russian а often contrasts one thing with another without the stronger but feeling of но.

Very roughly:

• а = whereas / while / and as opposed to
• но = but

In this sentence, the speaker is simply contrasting two types of clothing, so а не is the most natural choice.

Yes, it could. Russian word order is flexible.

This sentence starts with В спортзал to set the scene or topic: As for going to the gym...

A more neutral order would be:

• Я обычно хожу в спортзал в толстовке, а не в пиджаке.

Both are correct, but the original version gives a bit more emphasis to going to the gym as the context.

Russian often moves elements around for emphasis, topic, or rhythm rather than because of a strict fixed word order like English.

Not strictly. Russian often omits subject pronouns when the verb already makes the subject clear.

• Обычно хожу в спортзал... is possible
• Я обычно хожу в спортзал... is also possible

Here я is included for clarity or mild emphasis. It can help make the statement feel more personal or contrastive.

Since хожу already clearly means I go, the pronoun is optional grammatically.

Because Russian uses the present tense to talk about regular habits, just like English does.

• я обычно хожу = I usually go
• он часто читает = he often reads
• мы редко ездим = we rarely go / travel

So even though the action happens repeatedly over time, Russian does not need a special tense for that here. The adverb обычно already shows that it is a habit.

Not in this sentence. With спортзал, Russian normally says в спортзал because it is a physical place/building you go into.

• в спортзал = to the gym
• в спортзале = in the gym

You might use на with a different noun, for example:

• на тренировку = to training / to a workout session

So:

• Я иду в спортзал. = I’m going to the gym.
• Я иду на тренировку. = I’m going to training / to a workout.

Those are related, but not identical, expressions.

Yes. Even though the Russian literally uses ходить в чём-то (to go in something), the natural English translation is often wear.

So depending on context, good English versions could be:

• I usually go to the gym in a sweatshirt, not a blazer.
• When I go to the gym, I usually wear a sweatshirt, not a blazer.

This is a good example of where you should understand the Russian structure, but not translate it too mechanically into English.